1. Field of the Invention
The present invention relates to a common mode choke coil preferably for use in a transmission line for a high frequency signal.
2. Description of the Related Art
In high-speed interfaces such as a USB (Universal Serial Bus) and an HDMI (High Definition Multimedia Interface), there has been used a “differential transmission system” in which signals whose phases are 180° different from each other are transmitted on a pair of signal lines (=parallel lines). In the differential transmission system, radiation noise and external noise are cancelled on the parallel lines, and hence these noises are not apt to exert an influence. However, in reality, especially on signal lines for the high-speed interface, a common mode noise current ascribed to the asymmetry of the signal lines is generated. A common mode choke coil is thus used for the purpose of suppressing this common mode noise.
As disclosed in FIGS. 1A and 1B of Unexamined Japanese Patent Publication No. 2003-068528, FIGS. 2A and 2B of Unexamined Japanese Patent Publication No. 2008-098625 and the like, the common mode choke coil is typically configured as a small-sized laminated chip component provided with two coils (primary coil, secondary coil) wound in the same direction. Here, the primary coil and the secondary coil are arrayed in a laminating direction inside a laminated element body.
FIG. 18 is a sectional view of the common mode choke coil shown in Unexamined Japanese Patent Publication No. 2003-068528. This common mode choke coil has a structure provided with two coils (laminated coils) 2, 3 which are coaxially wound and axially disposed separately in a laminated element 1, and a leader and a trailer of each of the coils 2, 3 are extracted to the end surface of each side of the laminated element 1 and connected to an external electrode.
However, a coupling degree between the primary coil and the secondary coil is difficult to make high just by simply arraying the primary coil and the secondary coil in the laminating direction inside the laminated element body. When the coupling degree between the primary coil and the secondary coil is low, an insertion loss of a normal mode signal increases. On the other hand, when the primary coil and the secondary coil are arranged close to each other so as to make the coupling degree high, a capacitance (stray capacitance) generated between the primary coil and the secondary coil increases. When this capacitance increases, differential impedance of the common mode choke coil decreases, and becomes unable to be matched with impedance of the balanced transmission line.
Further, in the structure where the primary coil and the secondary coil are arrayed in the laminating direction inside the laminated element body, there occurs displacement of a formed position of a coil pattern or displacement of lamination of sheets due to a process problem. Moreover, when the coils are mounted on a printed wiring board, a capacitance between the primary coil and a ground conductor and a capacitance between the secondary coil and the ground conductor becomes unbalanced due to a structural problem such as a difference in coupling amount between each coil and the ground conductor. For this reason, the symmetry between the primary coil and the secondary coil cannot be ensured, leading to conversion of the common mode noise to the normal mode signal (noise). That is, the ability to remove the common mode noise is degraded.
Further, although a magnetic body may be used as the laminated element body, since the magnetic body has relatively large frequency dependence, a loss of the normal mode signal especially in a high frequency band is apt to become large. Moreover, a sufficient coupling value between the primary coil and the secondary coil cannot be obtained especially in the high frequency band, and the loss of the normal mode signal is apt to become large.